Cellular O2 sensing is an important biological process in health and disease. The ability to detect and respond to hypoxia is required for embryonic development, for transition from placental to lung respiration at birth, and for systemic oxygen homeostasis throughout life. In previous funding cycles we discovered that mitochondria regulate the signaling of hypoxia to the cell through the release of reactive oxygen species (ROS) from complex III, which then activate cellular protective and adaptive responses. We also developed genetic tools for assessing subcellular redox signaling and for modifying ROS generation from complex III. Inhibition of hypoxia-induced mitochondrial ROS signals by genetic deletion of the Risked Iron-Sulfur Protein (RISP) or expression of an H2O2 scavenger in the mitochondrial intermembrane space abrogated hypoxia responses in diverse cell types. Unexpectedly, deletion of RISP in the adult mouse heart induced profound remodeling characterized by a ~2.5-fold increase in heart weight and thickening of the ventricular walls, with no evidence of cellular hypertrophy. Cardiomyocyte diameter, cell morphology, and mitochondrial ultrastructure were unchanged, while immunostaining for markers of cellular proliferation (Ki67) revealed profuse distributions of labeled cells, consistent with the generatio of new cardiomyocytes. Gene array analysis during remodeling identified activation of pathways involving cytoskeletal reorganization but no signature characteristic of cardiac hypertrophy. We hypothesize that ROS signals are generated by cardiac mitochondria in response to the decrease in myocardial PO2 that occurs as the neonatal heart transitions from a fetal glycolytic program into the adult oxidative phenotype. We postulate that these ROS signals trigger the cell cycle arrest that develops by postnatal day 7. Deletion of RISP, which is required for hypoxia-induced ROS signaling, abrogates the ROS signaling and permits adult cardiomyocytes to re-enter the cell cycle. We will test these hypotheses using genetic models to manipulate mitochondrial ROS generation and signaling, identify early genes involved in the activation of this response, and determine the cellular mechanisms underlying the regulation of proliferative arrest by heart mitochondria. Finally, we will test whether reactivation of proliferation in hearts damaged by ischemia-reperfusion can be induced by modulating complex III function, allowing rescue of function through the generation of new cardiomyocytes. The results of these studies could provide a major advance in our understanding of how cell proliferation in the heart is controlled, and carry profound potential clinical significance in terms of the treatment of ischemi injury in the heart.